One type of adhesive is a dry adhesive, which binds two surfaces or objects together without being required to flow and wet to the adherend, as do adhesives such as glues, cements, and sealants. Dry adhesive may exploit an aggregate van der Waals force between the dry adhesive and the contact surface when the adhesive is in intimate contact with the contact surface.
Van der Waals forces are short-range intermolecular forces, i.e., forces between molecular entities, other than those due to chemical bonds or due to electrostatic interactions with ions or ionic groups. Van der Waals forces include attractive forces due to the electrical attraction between permanent and/or induced dipoles of the molecules. Van der Waals forces are weak compared to chemical bonds and are negligible at separation distances greater than about 1 nm (nanometer). Hence, van der Waals forces between solid-phase molecules only occur upon intimate contact (less than about 1 nm) between molecules.
Though van der Waals forces are weak, the aggregate total of van der Waals forces between solid surfaces may be macroscopically significant, provided that there is sufficiently intimate contact. Many species of gecko lizards have numerous tiny spatulae on their feet that may come into intimate contact with a surface to form an aggregate van der Waals attractive force, sufficient to permit the gecko lizard to support itself on shear, smooth surfaces (such as vertical glass surfaces). The specialized toe pads of the gecko lizard (and similar structures along the legs of some spiders) inspired dry adhesives based on flexible micro- or nano-structures.
One type of dry adhesive is based on a carbon-nanotube forest. The carbon nanotubes of the forest are a group of carbon nanotubes with aligned longitudinal axes (substantially parallel in a bundle). The carbon-nanotube forest superficially resembles a nanoscale bamboo forest where the carbon nanotubes are the ‘trees.’ Some carbon-nanotube forests have very thin, long carbon nanotubes, capable of sufficiently conforming to a surface to provide an attractive van der Waals force, exceeding 10 N/cm2 (newtons per square centimeter). Since van der Waals forces between the individual carbon nanotubes and the contact surface fundamentally are contact forces, the total number of atomic contacts and, hence, the contact area, substantially determines the total attractive force between the individual carbon nanotubes and the contact surface.
Intimate contact of the carbon-nanotube-forest adhesive with a surface typically is achieved by compressing such adhesive onto the surface (similar to pressure sensitive adhesive). Once the carbon-nanotube-forest adhesive is in intimate contact with the surface, the adhesive may be removed from the surface by applying an external force that overwhelms the aggregate van der Waals force. Generally, carbon-nanotube-forest adhesives are more resistant to shear forces than to normal forces. Shear forces tend to stress individual van der Waals contacts in parallel while normal forces tend to stress individual van der Waals contacts serially. Hence, normal forces may cause the tips of the carbon nanotubes to peel away one van der Waals contact at a time.
The force to separate a carbon-nanotube-forest adhesive from a surface may be at least as great as the force that can be safely supported by the adhesive. The separation force may be applied manually or may be applied by an actuator. For movable systems, such as use of the adhesive on a robot, an actuator sized to apply the needed separation force may add extra complexity, weight, and power requirements.